Oscillators based on lightly damped microscopic mechanical resonators are well known for their ability to produce stable, low-noise frequency outputs. While these characteristics make them valuable in communication systems as stable timing frequency references, they also make them attractive for use as sensors. A resonant sensor, by definition, is an oscillator whose output frequency is a function of an input measurand. In other words, the output of a resonant sensor corresponds to the shift in resonant frequency of a mechanical microstructure that gets tuned in accordance to a change in a physical chemical quantity to be measured. The quasi-digital nature of the output signal in such sensors, along with the high sensitivity and stability of the frequency shifted output signals, have resulted in wide spread use of such micromachined resonant sensors for numerous applications ranging from bio-molecular and chemical diagnostics, to high-precision force, mass, strain and even charge sensing.
As a particular case of resonant sensors, there has been an increased interest over the past few years in the development of high precision micromachined ‘all-silicon’ resonant microaccelerometers. See for example: U.S. Pat. No. 5,969,249; U.S. Pat. No. 4,851,080; US20110056294; CN101303365. This interest has been triggered due to the recent growth in demand for miniature high precision motion sensors within the aerospace, automotive and even the consumer-electronics markets. Resonant microaccelerometers fabricated using silicon micromachining techniques present a number of significant advantages, the biggest being economy. These silicon resonant microaccelerometers not only boast improved sensitivity and resolution relative to their more traditional capacitive detection based counterparts with similar device footprints, but have also been shown to provide enhanced dynamic range making them ideal candidates for potential application in numerous motion sensing applications within the identified markets.
However, most of these sensors still remain uniaxial or biaxial, consequently restricting their functionality and practical applicability to those applications that do not demand sophisticated three dimensional (3D) motion control. Whilst three uniaxial, orthogonally oriented resonant microaccelerometers could potentially be employed for a precise three dimensional frequency shifted acceleration motion read out, such implementations correspondingly increase the cost, size and power requirements of the device.
It is an object of the present invention to provide a micromachined silicon resonant accelerometer that allows for two and three dimensional acceleration read out using only a single suspended proof mass.